Linux builds were left behind on the Qt transition, presumably because
our Ubuntu CI image does not support Qt6.
Enable Qt6 by default and explicitly disable it for slower or more
conservative Linux distros.
Drop experimental status for Qt6, because we are using it to build
official Windows and macOS releases.
Run tests when nocheck is not set in DEB_BUILD_OPTIONS and skip
tests and building tests when nocheck is set.
Also make the check's style in sync with the official Debian package.
We keep our various packaging assets in the "packaging" directory. Move
the Debian assets there. dpkg-buildpackage doesn't seem appear to have a
"debian directory path" option, but symlinking worked in my test
container.
João Valverde